1. Field of the Invention
This invention pertains to solar power supplies, particularly for use in satellites and other spacecraft.
2. Description of the Prior Art
Earth-orbit satellites have been provided with solar power supplies comprising arrays of semiconductor devices that directly convert sunlight to electrical energy. The semiconductor devices, called "solar cells", are typically arrayed on rigid substrates affixed to exterior portions of the satellites. On present U.S. satellites, solar cells are typically mounted in arrays on rigid honeycomb substrates. However, proposals have been made to provide future satellites with power supplies comprising arrays of solar cells mounted on flexible substrates that are foldable into a pleated or accordion-like configuration for stowage during satellite flight from earth to orbit. For a solar power supply as proposed prior to the present invention, attachment of individual solar cells to a flexible dielectric substrate would be accomplished by welding the solar cells to metallic traces formed by a photolithographic process on a surface of the dielectric substrate.
A solar power supply on a spacecraft for journeying into "deep space" (i.e., into orbital or non-orbital flight more than 300 miles from the earth) would be subjected to very rapid fluctuations in temperature over a wide temperature range. As the power supply changes orientation with respect to the sun, temperatures experienced by the power supply might fluctuate from near absolute zero to upwards of 300.degree. C. in time intervals on the order of ten seconds depending upon factors such as orbit inclination.
A solar power supply has previously been proposed in which metallic traces would be formed on a first sheet of flexible dielectric substrate material, with a second sheet of flexible dielectric substrate material being disposed overlying the metallic traces on the first sheet. Holes would be provided in the overlying second sheet of substrate material, and solar cells would be positioned in an array over the holes in the overlying second sheet. Electrical terminals of the solar cells would be welded through the holes in the overlying second sheet to the metallic traces on the underlying first sheet. The weld joints between the electrical terminals of the solar cells and the electrically conductive traces on the underlying first substrate sheet, however, would be subjected to severe and repeated mechanical stresses during temperature cycling in deep space because the electrical terminals of the solar cells and the metallic traces on the underlying first substrate sheet would generally have coefficients of thermal expansion that are radically different from the coefficient of thermal expansion of the dielectric substrate material. Such stresses would in time tend to rupture the weld joints.
It has been proposed to relieve mechanical stresses that would otherwise adversely affect the above-described weld joints between solar cells and metallic traces on the underlying first substrate sheet by using an elastomer for adhesively bonding the solar cells to the overlying second substrate sheet, so that the weld joints would function primarily as electrical contacts and would not serve as the primary mechanical connections between the solar cells and the substrate. However, elastomeric adhesives tend to lose bonding strength at high temperatures and to become rigid at low temperatures. Thus, elastomeric adhesive bonding cannot be relied upon at the temperature extremes of deep space to provide the mechanical bonding required to secure solar cells to a substrate. Furthermore, differences between the coefficients of thermal expansion of the substrate material and the solar cell semi-conductor material would impose stresses directly on the semi-conductor material, thereby tending to fracture the semiconductor material during temperature cycling.
Until the present invention, there has been no technique for attaching an array of solar cells to a flexible dielectric substrate by means of an electrically conductive joint that remains substantially unstressed during temperature cycling over the range of temperatures experienced in deep space.